Nova db/api.py docstring cleanups...

We no longer use LazyPluggable for the db_backend, and
dbapi_use_tpool options. These now come from
nova.openstack.common.db.api.

Also, removed doc about enable_new_services which seemed a bit
lonely without the rest and probably isn't needed since the
config option help explains it well enough.

Change-Id: I14d00e013a1a0c0dfc80e0bba8ad82e97940d6ac
Merge "Nova db/api.py docstring cleanups..."